JP2003-106140A, published by the Japan Patent Office in 2003, discloses a method of diagnosing the deterioration of a catalyst supported on the surface of a diesel particulate filter (DPF) that traps particulate matter contained in the exhaust gas of a diesel engine.
The DPF is regenerated by burning the trapped particulate matter with high-temperature exhaust gas. When an oxidation catalyst is coated onto a bed which constitutes the filter of the DPF, an oxidation reaction is promoted while the particulate matter deposited on the filter is burned during regeneration, causing the bed temperature of the filter to rise. As a result of this temperature-raising effect, the extent to which the temperature of the exhaust gas must be raised to perform a DPF regeneration operation can be reduced.
However, the oxidation catalyst deteriorates as the filter is regenerated repeatedly. When the oxidation catalyst deteriorates, the bed temperature of the filter cannot be raised sufficiently during regeneration of the DPF, and hence regeneration of the DPF may not be completed.